There are some options for this.
one would be to make a periodic trigger which every 2 seconds picks each unit of type barracks and creates a unit of type footman at the position of the picked unit.
another one would be to set the build time for the footman to 2 second and the build cost to 0 and then order the unit to buid a footman upon creation of the building and recreate it every time a footman is finished.
both option are fairly easy triggers, even for beginners, if it's still not clear, leave a message, i can provide a dummy map for you during the weekend.